What are Iterators and Generators?

const range = {
  from: 1,
  to: 5,
  [Symbol.iterator]() {
    let current = this.from;
    const last = this.to;
    return {
      next() {
        if (current <= last) {
          return { value: current++, done: false };
        }
        return { done: true };
      }
    };
  }
};

for (const num of range) {
  console.log(num); // 1, 2, 3, 4, 5
}

function* numberGenerator() {
  yield 1;
  yield 2;
  yield 3;
}

const gen = numberGenerator();
gen.next(); // { value: 1, done: false }
gen.next(); // { value: 2, done: false }
gen.next(); // { value: 3, done: false }
gen.next(); // { value: undefined, done: true }

// Spread into array
[...numberGenerator()]; // [1, 2, 3]

// Infinite sequence
function* fibonacci() {
  let [a, b] = [0, 1];
  while (true) {
    yield a;
    [a, b] = [b, a + b];
  }
}

// Take first 10
const fib = fibonacci();
const first10 = Array.from({ length: 10 }, () => fib.next().value);

// Async generator
async function* fetchPages(urls) {
  for (const url of urls) {
    const response = await fetch(url);
    yield await response.json();
  }
}

for await (const page of fetchPages(urls)) {
  console.log(page);
}